This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. whether remediation of vitamin D insufficiency will improve physical function and the potential mechanisms involved are unclear. Previous vitamin D supplementation trials, primarily in falls clinic or home-bound individuals, have produced mixed results on muscle strength and physical performance;however, most trials did not specifically recruit individuals who were vitamin D insufficient nor is the potential mechanism of action understood well enough to appropriately select those individuals most likely to benefit. In order to determine if vitamin D supplementation will improve physical function, a trial is needed among vitamin D insufficient individuals in whom vitamin D supplementation is most likely to be beneficial. The primary goal of this feasibility pilot study is to collect key information to help design a randomized trial to determine whether vitamin D supplementation will delay declines in physical function. The pilot study will help us determine cost-effective strategies for identifying persons at high risk for functional decline with insufficient vitamin D levels, determine the serum vitamin D response to a regimen designed to attain sufficient vitamin D levels, and provide preliminary data of key functional measures (balance, short physical performance battery (SPPB) and muscle power) for the future larger study design. The secondary goal is to begin to examine potential mechanisms by which vitamin D supplementation may enhance physical performance and muscle contractility. Before a definitive trial is designed to answer these questions, we must first address important questions regarding feasibility, sample size, and preliminary evidence of effects.